The present invention relates to digital audio reproduction equipment. Since the introduction of digital audio equipment in the early eighties, critical listeners have often complained of an undefined "harshness" and "uncertainty" in the upper treble region of the sound reproduced by digital audio equipment. The source for the "harshness" was determined to be the result of an uncertainty in the start and stop of
a) the timing signal, the `work clock` of the digital-to-analog converter, (DAC), and
b) the data bits in the serial digital interface to equipment external to the reproduction equipment.
In the industry this type of timing uncertainty is known as "Phase Jitter" of digital signals and in many other applications it is of no concern. The source of this unwanted jitter was determined to be:
a) mechanical resonance of the compact disc (CD) itself, defects of the CD such as drop-outs, disc eccentricity, and errors in focus and tracking;
b) a basic design problem of large scale integrated (LSI) circuits used in the equipment to process digital signals.
The mechanical resonance is an inherent characteristic of the CD; a typical CD has a self-resonant frequency in the midrange of human hearing at about 850 Hz. The resonance is of a very high "Q", i.e. the resonance bandwidth is very narrow. The resonance occurs when sonic energy in the range of the resonant frequency is reproduced in the vicinity of the CD player. The acoustic feedback causes the disc to vibrate at its resonant frequency. The vibration of the CD induces the focus servo of the optical read head to continuously refocus, and thereby demanding extra power from the power supply to perform the refocusing operation. Thus the focus servo modulates the output of the power supply, thereby causing unwanted electrical reactions by other electronic components of the CD player.
The industry has addressed this problem and reduced the harshness caused by the resonant frequency of the CD by introducing fly-wheel weights and damping devices to be placed on the CD during operation. Such a procedure changes the vibration of the disc and changes the recognized `harshness` and `uncertainty`. However, changing the mechanical response characteristic of the CD does not solve the problem, it reduces only one source of the distorting signals, the acoustic feedback via a vibrating CD.